Variadic ops optimization

[quasilyte]

x + y + z translated to:

eval x
eval y
concat 2
eval z
concat 2

But it is better to emit:

eval x
eval y
eval z
concat 3

It also makes it easier to constant fold string concatenation sequences. Note that lisp.Concat(x, y, z) generates optimal code.

This implementation is bad, but can give starting approximation:

func weakenConcat(form *sexp.InstrCall) sexp.Form {
	switch len(form.Args) {
	case 0:
		return sexp.Str("")
	case 1:
		return form.Args[0]
	}
	// This code should be re-written.
	args := make([]sexp.Form, 0, len(form.Args))
	for _, arg := range form.Args {
		arg = ReduceStrength(arg)
		if arg, ok := arg.(*sexp.InstrCall); ok {
			if bytes.Equal([]byte("concat"), arg.Instr.Name) {
				args = append(args, arg.Args...)
				continue
			}
		}
		args = append(args, arg)
	}
	form.Args = args
	form.Instr = instr.Concat(len(args))
	return form
}